The automatic data collection (ADC) arts include numerous systems for representing information in machine-readable form. For example, a variety of symbologies exist for representing information in bar code symbols, matrix or area code symbols and/or stacked symbols. A symbology typically refers to a set of machine-readable symbol characters, some of which are mapped to a set of human-recognizable symbols such as alphabetic characters and/or numeric values. Machine-readable symbols are typically composed of machine-readable symbol characters selected from the particular symbology to encode information. Machine-readable symbols typically encode information about an object on which the machine-readable symbol is printed, etched, carried or attached to, for example, via packaging or a tag.
Bar code symbols are a common one-dimensional form of machine-readable symbols. Bar code symbols typically comprise a pattern of vertical bars of various widths separated by spaces of various widths, with information encoded in the relative thickness of the bars and/or spaces, each of which have different light reflecting properties. One-dimensional bar code symbols require a relatively large space to convey a small amount of data.
Two-dimensional symbologies have been developed to increase the data density of machine-readable symbols.
Some examples of two-dimensional symbologies include stacked code symbologies. Stacked code symbologies may be employed where length limitations undesirably limit the amount of information in the machine-readable symbol. Stacked code symbols typically employ several lines of vertically stacked one-dimensional symbols. The increase in information density is realized by reducing or eliminating the space that would typically be required between individual bar code symbols.
Some other examples of two-dimensional symbologies include matrix or area code symbologies (hereinafter matrix code). A matrix code machine-readable symbol typically has a two-dimensional perimeter, and comprises a number of geometric elements distributed in a pattern within the perimeter. The perimeter may, for example, be generally square, rectangular or round. The geometric elements may, for example, be square, round, or polygonal, for example hexagonal. The two-dimensional nature of such a machine-readable symbol allows more information to be encoded in a given area than a one-dimensional bar code symbol.
The various above-described machine-readable symbols may or may not also employ color to increase information density.
A variety of machine-readable symbol readers for reading machine-readable symbols are known. Machine-readable symbol readers typically employ one of two fundamental approaches, scanning or imaging.
In scanning, a focused beam of light is scanned across the machine-readable symbol, and light reflected from and modulated by the machine-readable symbol is received by the reader and demodulated. With some readers, the machine-readable symbol is moved past the reader, with other readers the reader is moved past the machine-readable symbol, and still other readers move the beam of light across the machine-readable symbol while the reader and machine-readable symbol remain approximately fixed. Demodulation typically includes an analog-to-digital conversion and a decoding of the resulting digital signal.
In imaging, the machine-readable symbol reader may flood the machine-readable symbol with light, or may rely on ambient lighting. A one-dimensional (linear) or two-dimensional image capture device or imager such as a charge coupled device (CCD) array captures a digital image of the illuminated machine-readable symbol, typically by electronically sampling or scanning the pixels of the two-dimensional image capture device. The captured image is then decoded, typically without the need to perform an analog to digital conversion.
A two-dimensional machine-readable symbol reader system may convert, for example, two-dimensional symbols into pixels. See, for example, U.S. Pat. No. 4,988,852 issued to Krishnan, U.S. Pat. No. 5,378,883 issued to Batterman, et al., U.S. Pat. No. 6,330,974 issued to Ackley, U.S. Pat. No. 6,484,944 issued to Manine, et al., and U.S. Pat. No. 6,732,930 issued to Massieu, et al.
Scanning type machine-readable symbol readers typically employ a source of coherent light such as a laser diode to produce a beam, and employ a beam deflection system such as an rotating or oscillating mirror to scan the resulting beam across the machine-readable symbols. Conventional laser scanning systems employ progressive symbol sampling. The cost of the beam deflection systems employed in laser scanners can be reduced, and the resistance to shock of such systems can be increased by employing a higher symbol scan rate. However, the use of a higher symbol scan rate in conventional systems requires a higher sampling rate, and thus a more complex and expensive data interface. Thus, it can be appreciated that a scanning type machine-readable symbol reader with a high scan rate and a reduced sampling rate is desirable.